This guide describes the suggested procedures for the preparation, testing, and certification of reference materials (RMs) to be used in the calibration, verification, and control of methods used to characterize the chemical composition of metals, ores, and related materials.

Certified reference materials are frequently rare and valuable commodities requiring investment of considerable cost and production time. They are frequently available for only a limited portion of a user' range of interest.

When comparative analytical methods are employed, appropriate CRMs are often unavailable for calibration. In this case, the use of RMs is indicated as the alternative choice.

The use of uncertified homogeneous materials is appropriate for control chart programs where relative data consistency is being monitored. The use of CRMs for such purposes is often a misuse of valuable CRM stocks, especially when uncertified materials of suitable homogeneity are available. For information on the use and misuse of CRMs, see ISO Guide 33 and NBS Special Publication 260-100.⁵

Use CRMs and RMs with caution in the validation of analytical methods. The danger involves a potential for undetected systematic error, since the same methodology being validated may have been used to establish the values for the CRMs or RMs. For more information on the use of CRMs in the validation of analytical methods, see NIST Special Publication 829.⁶